Acoustic projector pressure release and equalization system



March 17, 1970 A. E. WALLEN ETAL 3,501,741

ACOUSTIC PROJECTOR PRESSURE RELEASE AND EQUALIZATION SYSTEM Filed NOV. 15, 1968 Z VE'N'T'UFPE 13. E}. LLIHLLIEN F Ll. LL/H/ 'T'E'HE'HD EH 9. W was /Z7"'7"U NEH 3,501,741 ACOUSTIC PROJECTOR PRESSURE RELEASE AND EQUALIZATION SYSTEM Albert E. Wallen, Winston-Salem, and Paul L. Whitehead, Burlington, N.C., assignors to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York Filed Nov. 15, 1968, Ser. No. 776,141 Int. Cl. H04r 1/44 US. Cl. 3408 10 Claims ABSTRACT OF THE DISCLOSURE In an acoustic projector, a baffile plate having a plurality of slidably mounted, spring-biased pistons is mounted between a gas filled pressure release chamber and a liquid filled acoustic generator chamber. The pressure release chamber is pressurized to a preestablished level so that the slidably mounted and spring-biased pistons present a certain compliance characteristic to the acoustic generator chamber. A pressure regulator system monitors and maintains the pressure within the pressure release chamber always at the optimum pressure above environmental pressure for the desired complance characteristic.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to an acoustic transducer for transmitting or receiving sounds in a fluid. In acoustic transducers, a problem exists in that when sound waves are generated to be transmitted in one direction, it is desirable to suppress acoustic vibrations tending to travel in the opposite direction in order to prevent acoustical disturbances leading to generation of harmonics, inefiicient operation and other problems. A standard technique, known as pressure release, is employed in which a medium having a relatively low acolustic impedance is located in a position opposed to the desired direction of radiation from the transducer. compressional waves are reflected from the low acoustic impedance and add to the waves traveling in the desired direction and thereby increase the efiiciency of the transducer.

Description of the prior art In the past, a number of different techniques have been developed for relieving back pressure and providing pressure release in underwater acoustic projectors. Two of the most commonly used systems have employed either a compliant air filled bag or metal relief tubes. Compliant air bags have been found unsatisfactory principally because of rapid deterioration in the material comprising the bag as well as changes in the compliant characteristics over a period of time. Metal relief tubes are generally unsatisfactory for providing pressure release at low frequencies because high amplitudes of vibration cause the metal tubes to fatigue and crack.

A second problem which exists in the prior art is that of adapting an acoustic pressure relief system for operation at a number of different environmental pressures. That is, a compliant air filled bag or metal relief tubes would have certain preselected compliant characteristics only at a preselected environmental pressure. If the environmental pressure is changed, such as by operating at a greater depth, the compliant characteristics of the bag or tubes would no longer be such as to provide maximum efficiency in the transducer.

SUMMARY OF THE INVENTION An object of the invention is a new and improved pressure release system for an acoustic projector or receivnited States Patent O i 3,501,741 Patented Mar. 17, 1970 "ice ing transducer. In one embodiment of the system, a bafile plate having a plurality of apertures therein is mounted perpendicular to the direction of travel of compressional Waves which it is desired to suppress. A plurality of resiliently biased pistons are respectively mounted in the apertures to form a piston array presenting a characteristic acoustic impedance to suppress the compressional waves incident thereon.

BRIEF DESCRIPTION OF THE DRAWING The nature of the present invention and its various advantages will appear more fully by referring to the following detailed description in conjunction with the appended drawing, in which:

FIG. 1 is a cut-away view of an acoustic projector construction in accordance with the invention;

FIG. 2 is a cross-sectional view of the pressure release pistons of the acoustic projector shown in FIG. 1; and

FIG. 3 is a cut-away view, looking toward the acoustic generator chamber, of the baffle plate and pressure release pistons shown in FIG. 1.

DETAILED DESCRIPTION Referring to FIG. 1, the acoustic projector comprises an elongated cylindrical shell 11 having a plurality of stabilizing fins 12 attached to one end. Mounted at the opposite end of the shell 11 from the fins 12 is a rounded flexible membrane 13, constructed, for example, of rubber. Acoustic vibrations generated within the projector are transmitted into the surrounding fluid through the rubber diaphragm 13.

Formed within the shell 11 .is an acoustic signal generator chamber designated generally as 14. The generator comprises a pair of segmented magnetic pole pieces 15 and 16 formed of a permanent magnet material surrounding a central soft iron core 17. A sound generating piston 18 is mounted upon a pair of crossed tuning bars 22 and 23 so that the piston is adjacent the soft iron core 17 in axial alignment therewith, and spaced a preselected distance therefrom. A wound coil of wire 19 is carried within the piston 18 so that when the coil is excited by an alternating electrical current (by means not shown), the piston moves alternately toward and away from the soft iron core 17. To insure operation at a preselected frequency of vibration, the pair of crossed tuning bars 22 and 23, having a certain size and shape so that they resonate at the chosen frequency, are attached to the end of the piston 18. The soft iron core 17 and magnetic pole pieces 15 and 16 are attached to a mounting plate 24 fixed within the cylindrical shell 11 and having a plurality of apertures 25-25 formed therein.

The opposite end of' the acoustic generator chamber 14 from the rubber diaphragm 13 is closed by a baffle plate 26 which is substantially perpendicular to the desired direction of radiation from the transducer. The baffle plate 26 has a plurality of cylindrical apertures formed therein with a slidably mounted, spring-biased piston assembly 27 located in each aperture. A first surface of each piston is in communication with the acoustic generator chamber 14 while the second surface of each piston communicates with a gas filled pressure release chamber 28. The acoustic generator chamber 14 is filled with a viscous fluid 40, such as oil, all the way from the rubber diaphragm 13 to one side of the bafile plate 26. All vibrations generated within the fluid 40 by the movement of the piston 18 are communicated to both the rubber diaphragm 13 and to the baflie plate 26, including the first surfaces of the pistons within the pis ton assemblies 2727.

Referring to FIG. 2, the baffle plate 26 comprises a thick metal plate having first and second surfaces and a plurality of apertures therein to receive the piston assemblies 2727. Each piston assembly includes a hollow cylinder 31 mounted within an aperture in the baffle plate 26, perpendicular to the surface of the baffle plate. A piston 32 with a first surface 29 and a second surface 30 and having a plurality of circumferential grooves 3333 formed therein is slidably mounted within the cylinder 31. An O-ring 34 is mounted within each of the circumferential grooves 33 to seal the walls of the piston 32 to the walls of the cylinder 31. A threaded cap 35 is screwed onto one end of the cylinder 31 to restrain movement of the piston 32 past a certain point. The cap 35 has a central aperture therein so that the viscous fluid 40 within the acoustic generator chamber 14 is in contact with the first surface 29 of the piston 32. The opposite end of the cylinder 31 is also closed by a threaded cap 36 having a central aperture therein so that the gas within the pressure equalization chamber 28 may come into contact with the second surface 30 of the piston 32. A helical spring 37 is mounted between the inwardly turned shoulders of the cap 36 and the second surface 30 of the piston 32. The tension of the spring, and hence the force which it applies to the piston 32 may be varied by screwing the cap 36 along the threads to various positions upon the cylinder 31.

Referring again to FIG. 1, the pressure release chamber 28 which is in communication with the opposite side of the bafile plate 26 from the viscous medium 40, is filled with a fiuid and preferably a gas, such as air, supplied from a high pressure gas cylinder 41 through a pressure regulator 42. The pressure in the supply cylinder 41 is very high, for example, in excess of 1000 pounds per square inch and is greater than the pressure at the maximum fluid depth at which the acoustic projector is to be operated. The regulator 42 includes a bellows-like pressure sensing means 43 which is in communication with the environmental pressure of the acoustic projector. A regulator may be used which is similar to the one described in an article entitled Differential Air Pressure Regulator published on pages 3232 of the April 1967 issue of the Western Electric Engineer. The regulator 42 includes valving apparatus which serves to deliver a flow of gas to the pressure release chamber at a pressure which is always held a preselected value above the environmental pressure. The pressure regulator system insures that when the environmental pressure of the acoustic projector is changed to vary the ambient pressure of the viscous medium 40, and hence the pressure on the first side 29 of the pistons 3232 the pressure on the second side 30 of the pistons 3232 is changed in the same proportion. The acoustic dampening and pressure release characteristics of the pistons therefore remain constant regardless of the depth at which the acoustic projector is operated.

In operation, the excitation coil 19 of the acoustic projector is energized at a certain frequency and at a preselected power level by an oscillator (not shown) to produce vibrations in viscous medium 40 and the rubber diaphragm 13. The oscillator may be carried by the projector and power coupled to the coil 19 via a cable through the pressure release chamber 28, the baflle plate 26 and the viscous medium 40. The oscillator signal could alternately be coupled to the acoustic projector from an external source via a cable from the surface.

The projector is initially tuned by screwing the threaded caps 3636 back and forth upon the threaded portion of the cylinder 3131 while the viscous medium is vibrating until the proper tension is exerted on the helical springs 37-37. Alternatively, the pressure of the viscous medium 40 may be increased until it is greater than the pressure in the pressure relief chamber 28 so that the pistons 3232 assume a normally central position (as shown in FIG. 2). For typical springs used this might be when the pressure in the acoustic generator chamber 14 is approximately 6 lbs/sq. in. greater than that in the pressure release chamber 28. When the springs 37-37 apply the correct opposing force to the pistons 3232, the piston array presents the optimum acoustic dampening characteristic to the viscous fluid 40 so that acoustic waves traveling in the opposite direction from the desired direction are suppressed by being addatively reflected. That is, with the proper pressure release characteristic, the compressional waves traveling in a direction opposite to the desired direction, i.e., toward the baffle plate 26, are reflected from the pistons assembly array within the bafile plate 26 in the proper phase relation so that they are added to the waves traveling in the desired direction, i.e., toward the rubber membrane 13. At the proper characteristic acoustic impedance, the efficiency of the acoustic transducer is at its maximum value and the vibrations of the rubber diaphragm 13 are at their peak amplitude so that the maximum amount of sound energy may be coupled into and transmitted through the fluid surrounding the acoustic projector.

It is to be noted that since the piston 18 is moving in both the forward and reverse directions, it is desirable that the resistance impedance to the motion of the fluid 40 be the same in both directions. That is, the piston array enables the fluid 40 to move in the reverse direction as easily as it can move in the forward direction toward the membrane 13. In this manner, the pressure release system prevents cavitation and other deleterious effects within the fluid 40.

The pistons are normally adjusted while the acoustic projector is above water and a typical pressure placed in the pressure release chamber 28 to tune the piston assembly array tuned for'maximum efficiency may be approximately 1.5 pounds above atmospheric pressure. When the projector is emersed at various depths in water, the differential pressure regulator 42 operates to maintain the pressure in the pressure release chamber 28 the preselected 1.5 pounds above the environmental pressure of the regulator. The changing air pressure on the regulating second sides 3030 of the pistons 3232 always maintain the acoustic pressure release characteristic of the piston assembly array at the previously adjusted value for maximum efficiency.

The use of a spring biased piston array in combination with a pressure regulator enables the acoustic projector to operate at maximum efficiency at any depth up to a thousand feet without surfacing for readjustment. Further, the power level of the acoustic projector is always maintained at maximum value and the acoustic level at which the projector may be efiiciently operated is limited only by the distance through which the pistons 32 may travel within the cylinders 31.

It is to be noted that the piston array and pressure regulator of the instant pressure release system may readily be adapted for use in a receiving type of acoustic transducer such as a hydrophone. The pressure release system will improve the efficiency and sensitivity of a hydrophone in the same general manner it improves an acoustic projector.

It is to be understood that the above-described embodiments are simply illustrative of the invention and that many other embodiments can be devised without departing from the scope and spirit of the invention.

What is claimed is:

1. A pressure release apparatus for use in an acoustic transducer, comprising:

a baffle plate having first and second faces and a plurality of apertures therethrough between the first and second faces, said bafiie plate being positioned substantially perpendicular to the desired direction of travel of compressional waves emanating from said acoustic transducer; and

a plurality of spring biased pistons respectively slidably mounted within each of the apertures in said baffle plate to form a piston array, said Piston array presenting a yieldable surface having a characteristic acoustic impedance to all compressional wave striking said array.

2. A pressure release apparatus for use in increasing the efiiciency of an acoustic projector having a compressional wave generator, comprising:

a bafile plate having first and second faces and a plurality of apertures therethrough between the first and second faces, said baffle plate being spaced away from said compressional wave generator in a direction opposed to the desired direction of projection of compressional waves and being substantially perpendicular to the desired direction of projection of compressional waves;

a plurality of pistons respectively slidably mounted in each of the apertures in said bafile plate to form a piston array; and

resilient means for biasing each of said pistons toward the first face of said bafiie plate adjacent said compressional wave generator to present a piston array thereto having a yieldable surface with a characteristic acoustic impedance to addatively reflect compressional waves traveling in a direction opposed to the desired direction of projection and thereby increase the efiiciency of the projector.

3. A pressure release apparatus as set forth in claim 2, wherein:

said resilient means for biasing each of said pistons may be adjusted as to degree of resilience to vary the characteristic acoustic impedance of the array for maximum efficiency in the projector.

4. A pressure release apparatus as set forth in claim 2, wherein:

the apertures in said baffle plate and said pistons are cylindrical and wherein said resilent means includes a coiled, helical spring.

5. A pressure release apparatus as set forth in claim 2, also including:

a pressure release chamber mounted adjacent to and in communication with the second face of said baflie plate; and

means for introducing a fluid into said chamber to assist said resilient means in providing a force upon said pistons to oppose the incident compressional waves and present a characteristic acoustic impedance thereto.

6. A pressure release apparatus as set forth in claim 5 wherein the fluid introduced into said chamber is a gas and including:

means for monitoring the environmental pressure of said acoustic projector and regulating said fluid introducing means to always maintain the fluid pressure in said chamber a preselected amount above the environmental pressure.

7. An acoustic projector for use in an environment characterized by an environmental acoustic impedance and pressure, comprising:

a vibrating element having compressional waves emanating therefrom in at least two directions; and

pressure release means operatively associated with said vibrating element and disposed transversely to one of said directions to suppress compressional waves traveling in said one direction;

6 said pressure release means including:

a bafiie plate having first and second faces and a plurality of apertures therethrough between the first and second faces, the first face being adjacent said vibrating element and the axes of 5 said apertures being along said one direction;

a plurality of pistons respectively slidably mounted in each of the apertures in said bafile plate to form a piston array; and

10 means for resiliently biasing each of said pistons in a direction opposed to said one direction to present a characteristic acoustic impedance to compressional waves traveling in said one direction to suppress said waves. S. An acoustic projector as set forth in claim 7 wherein said pressure release means also includes:

a pressure release chamber adjacent and in communication with the second face of said baffle plate; and

means for introducing a gas into said chamber and maintaining the pressure of said gas substantially equal to the environmental pressure of said acoustic projector so that said piston array always presents the proper characteristic acoustic impedance to compressional waves traveling in said one direction regardless of changes in the environmental pressure of said acoustic projector.

9. A pressure release apparatus for use in increasing the efiiciency of an acoustic receiver having a transducer therein, comprising:

a bafile plate having first and second faces and a plurality of apertures therethrough between the first and and second faces, said baflle plate being spaced away from said transducer in a direction opposed to the desired direction from which compressional waves are to be received and being substantially perpendicular to the direction of said waves;

a plurality of pistons respectively slidably mounted in each of the apertures in said baffle plate to form a piston array; and

resilient means for biasing each of said pistons toward the first face of said baffie plate adjacent said transducer to present a piston array thereto having a yieldable surface with a characteristic acoustic impedance to reflect compressional waves traveling in a desired direction of wave reception and thereby increase the efiiciency of the acoustic receiver.

10. A pressure release apparatus as set forth in claim 9, also including:

a pressure release chamber mounted adjacent to and in communication With the second face of said bafile plate; and

means for introducing a fluid into said chamber to assist said resilient means in providing a force upon said pistons to oppose the incident compressional waves and present a characteristic acoustic impedance thereto.

RODNEY D. BENNETT, ]R., Primary Examiner r BRIAN L. RIBANDO, Assistant Examiner 

